Engine exhaust systems for turbocharged engines commonly include the turbocharger arranged upstream in an exhaust flow direction from the exhaust aftertreatment devices (e.g., catalysts). Such an arrangement, while suitable for fast turbocharger response during some conditions, can lead to increased emissions during cold start conditions due to exhaust heat loss through the turbine of the turbocharger. Further, the exhaust backpressure created by the aftertreatment devices results in increased turbine outlet pressure, reducing the efficiency of the turbocharger.
Other attempts to address the issue of compromised emissions due to heat loss through the turbine include an aftertreatment device closely coupled to the engine. One example approach is shown by Bennet et al. in U.S. Pat. No. 8,276,366. Therein, a plurality of aftertreatment devices are coupled in a housing having multiple flow paths to allow flow of exhaust through one or more of the aftertreatment devices and a turbine of a twin turbocharger. Depending on operating conditions, the exhaust may flow through a turbine prior to flowing through one or more of the aftertreatment devices, or the exhaust may flow through one of the aftertreatment devices prior to flowing through a turbine.
However, the inventors herein have recognized potential issues with such systems. As one example, in every possible flow path in the housing of Bennet, exhaust always flows through at least one aftertreatment device after flowing through a turbine. Thus, the system of Bennet still suffers from the increased turbine outlet pressure that results from subsequent exhaust flow through downstream aftertreatment devices. As a further example, when a flow path is selected that routes exhaust from the engine directly to a turbine and then through one or more aftertreatment devices, it results in one of the aftertreatment devices (an oxidation catalyst) being bypassed altogether. Thus, at least in some examples, emissions may still be comprised. Further still, in Bennet, exhaust always flows through a turbine before flowing through a particulate filter, and thus particulate matter may impinge on the turbine blades, eventually leading to turbine degradation.
In one example, the issues described above may be addressed by a method for an exhaust system of an engine. The method includes, during a first condition, flowing exhaust gas through a turbine, from the turbine to at least one aftertreatment device, and then from the at least one aftertreatment device to atmosphere, and during a second condition, flowing exhaust gas through the at least one aftertreatment device, from the at least one aftertreatment device to the turbine, and then from the turbine to atmosphere. In this way, a flow path through the exhaust system may be selected that prioritizes turbine response (e.g., during the first condition) or that priorities rapid aftertreatment device light-off (e.g., during the second condition).
As one example, the first condition may be an engine acceleration event where a large torque increase is requested (e.g., during a vehicle launch), and thus the exhaust may be routed directly to the turbine to quickly provide the requested torque. The second condition may be engine cold start conditions where the one or more aftertreatment devices are below light-off temperature, and thus exhaust may be routed through the at least one aftertreatment device prior to traveling through the turbine. In both the first and second conditions, exhaust still flows through both the turbine and the at least one aftertreatment device, and thus no trade-off between emissions and turbine response is required. Further, by maintaining the turbocharger physically between the engine and the at least one aftertreatment device, packaging challenges that result from placing the aftertreatment devices before the turbocharger can be avoided. Further still, if the at least one aftertreatment device includes a particulate filter, by flowing exhaust through the particulate filter before the turbine, at least during some conditions, particulate matter impingement on the turbine may be reduced, increasing the life of the turbine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.